Source: Arsenal, page 35
Subsonic ammunition features a lighter charge and a slightly smaller bullet than regular rounds. This slows the bullets’ muzzle velocity below the speed of sound, reducing the sound of the weapon being fired and making it easier to silence.
Subsonic ammunition applies a –1 dice pool modifier on all Perception Tests to notice the weapon’s use or locate the firing position. This dice pool modifier increases to –2 if a silencer (see p. 311, SR4) or sound suppressor (see p. 312, SR4) is used. Reduce the weapon’s range by 20%.